The present invention relates to a process for introducing a portion withdrawn from at least one production charge of annular coated catalysts K into a reaction tube of a tube bundle reactor for the purpose of charging this reaction tube with a fixed catalyst bed suitable for performing a heterogeneously catalyzed partial gas phase oxidation of an organic starting compound.
Processes for heterogeneously catalyzed partial gas phase oxidation of organic starting compounds in fixed catalyst beds disposed in the reaction tubes of tube bundle reactors are known for the preparation of numerous industrial chemicals.
Examples of such heterogeneously catalyzed partial gas phase oxidations of organic compounds include the conversion of methanol to formaldehyde (cf., for example, CH-A 449 600 and CH-A 38 828), the conversion of propene to acrolein and/or acrylic acid (cf., for example, DE-A 23 51 151), the conversion of tert-butanol, isobutene, isobutane, isobutyraldehyde or the methyl ether of tert-butanol to methacrolein and/or methacrylic acid (cf., for example, DE-A 25 26 238, EP-A 092 097, EP-A 058 927, DE-A 41 32 263, DE-A 41 32 684 and DE-A 40 22 212), the conversion of acrolein to acrylic acid and of methacrolein to methacrylic acid (cf., for example, DE-A 25 26 238), the conversion of o-xylene and/or naphthalene to phthalic anhydride (cf., for example, EP-A 522 871) and the conversion of butadiene to maleic anhydride (cf., for example, DE-A 21 06 796 and DE-A 16 24 921), the conversion of C4 hydrocarbons such as 1-butene, 2-butene, butadiene and/or n-butane to maleic anhydride (cf., for example, GB-A 14 64 198 and GB-A 12 91 354), the conversion of indanes to anthraquinones (cf., for example, DE-A 20 25 430), the conversion of ethylene to ethylene oxide (cf., for example, EP-A 352 849, EP-A 352 850, EP-A 532 325, U.S. Pat. Nos. 5,155,242 and 5,262,551) or of propylene to propylene oxide (cf., for example, DE-B 12 54 137, DE-A 21 59 346, EP-A 372 972, WO 89/07101, DE-A 43 11 608), the conversion of propylene and/or acrolein to acrylonitrile (cf., for example, DE-A 23 51 151), the conversion of isobutene and/or methacrolein to methacrylonitrile (i.e. the term “partial oxidation” shall, in this document, also comprise partial ammoxidation, i.e. a partial oxidation in the presence of ammonia), the oxidative dehydrogenation of hydrocarbons or hydrocarbon derivatives (cf., for example, DE-A 23 51 151), the conversion of propane to acrylonitrile or to acrolein and/or acrylic acid (cf., for example, DE-A 101 31 297, EP-A 10 90 684, EP-A 608 838, DE-A 100 46 672, EP-A 529 853, WO 01/96270 and DE-A 100 28 582) etc.
While a full oxidation of an organic compound with molecular oxygen is understood in this document to mean that the organic compound is converted under the reactive action of molecular oxygen such that all of the carbon present in the organic compound is converted to oxides of carbon and all of the hydrogen present in the organic compounds to oxides of hydrogen, all different exothermic conversions of an organic compound under the reactive action of molecular oxygen are summarized in this document as partial oxidations of an organic compound.
In particular, in this document, partial oxidations shall be understood to mean those exothermic conversions of organic compounds under the reactive action of molecular oxygen in which the organic compound to be oxidized partially, after the conversion has ended, comprises at least one oxygen atom more in chemically bound form than before the partial oxidation was performed.
A tube bundle reactor is normally an apparatus which comprises a vertically arranged bundle of reaction tubes which is surrounded by a reactor jacket, both ends of the individual reaction tubes being open and the upper end of each reaction tube ending sealed into a passage orifice of an upper tube plate sealed at the top into the reactor jacket and the lower end ending sealed into a passage orifice of a lower tube plate sealed at the bottom into the reactor jacket, the exterior of the reaction tubes, the upper and the lower tube plate and the reactor jacket together delimiting the reaction tube surrounding space, and each of the two tube plates being spanned by a reactor hood having at least one orifice. In the performance of a heterogeneously catalyzed partial gas phase oxidation in such a tube bundle reactor, its reaction tubes are charged with a fixed catalyst bed (a fixed catalyst bed is introduced into its reaction tubes; a fixed catalyst bed is disposed in its reaction tubes) and a reaction gas input mixture which comprises the organic compound (organic starting compound) to be oxidized partially and molecular oxygen is fed in through the at least one orifice in one of the two reactor hoods, and the product gas mixture which comprises the target product which results through partial gas phase oxidation of the organic starting compound to be oxidized partially to the desired target product as it flows through the fixed catalyst bed disposed in the reaction tubes is removed via the at least one orifice of the other reactor hood, while at least one (generally liquid) heat exchange medium is conducted around the reaction tubes on the jacket side of the tube bundle reactor. Normally, in the case of use of at least one liquid heat exchange medium, it is conducted around the reaction tubes such that each of the two surfaces of the two tube plates facing one another is wetted by liquid heat exchange medium. The at least one (for example liquid) heat exchange medium is typically conducted into the reaction tube surrounding space with a temperature THin and back out of the reaction tube surrounding space with the temperature of THout.
The statement that the reaction tubes are sealed into the passage orifices in the upper and lower tube plate means that there is no means of passage for the heat exchange medium between the reaction tube outer wall and the bore wall (i.e. wall of the passage orifice or else shell of the passage orifice). Such a seal can be effected, for example, as described in DE-20 2006 014 116 U1.
In principle, the at least one heat exchange medium may also be conducted in gaseous form or in the boiling state through the reaction tube surrounding space. Examples of such tube bundle reactors and heterogeneously catalyzed partial gas phase oxidations performed therein are disclosed, for example, by EP-A 700 893, DE-A 44 31 949, WO 03/057 653, EP-A 16 95 954, WO 03/055 835, WO 03/059 857, WO 03/076 373, DE 699 15 952 T2, DE-A 10 2004 018 267, DE 20 2006 014 116 U1 and DE 10 2007 019 597.6, and also the prior art cited in the aforementioned documents.
In general, the components of the tube bundle reactor are manufactured from steel. Useful manufacturing steel is both stainless steel (for example of DIN materials number 1.4541 or 1.4571) and black steel or ferritic steel (for example DIN materials 1.0481, 1.0315 or material 1.0425). Frequently, all components of the tube bundle reactor are manufactured from the same steel type. In many cases, the reactor hoods are manufactured from ferritic steel and plated on their inner side with stainless steel. In some cases, the reactor jacket is also manufactured from a different steel type from the remaining part of the tube bundle reactor, since rolled steel can be used for its production.
In this document, the reaction tube surrounding space is defined as the space delimited by the exterior of the reaction tubes, the two tube plates and the reactor jackets together, within which the at least one (generally liquid) heat exchange medium is conducted. In the simplest manner, in the reaction tube surrounding space, only one (preferably liquid) heat exchange medium is conducted (such a procedure is also referred to as a one-zone method in the one-zone tube bundle reactor). It is typically fed to the reaction tube surrounding space at its upper or at its lower end with its entrance temperature THin through orifices in the reactor jacket, and conducted back out of the reaction tube surrounding space at the opposite end with an exit temperature of THout through orifices in the reactor jacket.
As a result of the exothermicity of the gas phase partial oxidations, during the performance of a heterogeneously catalyzed partial gas phase oxidation, THout≧THin (equality relates to the case of evaporative cooling). With the aid of a heat exchanger, heat is typically withdrawn from a portion or the entirety of the (preferably liquid) heat exchange medium conducted out of the reaction tube surrounding space before it is fed back to the reaction tube surrounding space with the temperature THin.
In the reaction tube surrounding space, the (preferably liquid) heat exchange medium can in principle be conducted around the reaction tubes in simple co- or countercurrent to the reaction gas mixture flowing within the reaction tubes. However, it can also be conducted around the reaction tubes in a meandering manner with the aid of corresponding deflecting plates, such that only over the entire reaction tube surrounding space does a cocurrent or countercurrent to the flow direction of the reaction gas mixture in the reaction tubes exist. When the heat exchange medium used is liquid under the use conditions, it should, appropriately from an application point of view, have a melting point in the range from 0 (or from 50) to 250° C., preferably from 120 to 200° C.
Useful such liquid heat exchange media include, for example, melts of salts such as potassium nitrate, potassium nitrite, sodium nitrite and/or sodium nitrate, and also melts of metals such as potassium, sodium, mercury and alloys of different metals. However, it is also possible to use ionic liquids (in which at least one of the oppositely charged ions comprises at least one carbon atom) or heat carrier oils (e.g. high-boiling organic solvents such as mixtures of Diphyl® and dimethyl phthalate). Useful gaseous heat exchange media include, for example, steam under elevated pressure or else flue gases. Evaporative cooling can, for example, also be undertaken with boiling water under pressure.
To improve the selectivity of target product formation, the heterogeneously catalyzed partial gas phase oxidation of an organic compound can also be performed as a multizone method (for example two-zone method) in a multizone tube bundle reactor (for example in a two-zone tube bundle reactor). In this case, within the reaction tube surrounding space, (for example two), essentially spatially separate (preferably liquid) heat exchange media (which are normally of the same type) are conducted (these may, for example, be separated by separating tube plates which have corresponding passage orifices for the reaction tubes and are inserted into the reaction tube surrounding space).
The reaction tube longitudinal section over which the particular (preferably liquid) heat exchange medium extends represents a temperature zone or reaction zone (the one-zone tube bundle reactor correspondingly has only one reaction zone).
Within the particular temperature zone, the (preferably liquid) heat exchange medium can be conducted as in the one-zone method (also relative to the flow direction of the reaction gas mixture). For the difference between THout and THin, the statements regarding the one-zone method apply in an essentially identical manner to the individual temperature zone.
A graphic distinction between a one-zone method and a two-zone method (between a one-zone tube bundle reactor and a two-zone tube bundle reactor) is shown schematically, for example, by the figures of DE 102007019597.6 and the figures of EP-A 1695954. Aside from these, multizone methods are described, for example, in documents EP-A 1734030, DE-A 10313214, DE-A 10313219, DE-A 10313211, DE-A 10313208 and in the prior art cited in these documents. They are advantageous in particular when a high loading of the fixed catalyst bed with the organic compound to be oxidized partially is selected. The loading of the fixed catalyst bed with reaction gas mixture or with one reaction gas mixture component is understood to mean the amount of reaction gas mixture or reaction gas mixture component in standard liters (I (STP); the volume that the corresponding amount would theoretically take up in gaseous form at 0° C. and 1 atm) which is conducted through one liter of fixed catalyst bed per hour (pure inert beds are not included).
The temperature THin of the at least one (preferably liquid) heat exchange medium in heterogeneously catalyzed partial gas phase oxidations of organic starting compounds is typically in the range from 200 to 500° C., frequently in the range from 250 to 400° C. and in many cases in the range from 250 to 310° C.
The working pressure in a heterogeneously catalyzed partial gas phase oxidation may be either below standard pressure (for example up to 0.5 bar; the reaction gas mixture is sucked through) or above standard pressure. Typically, the aforementioned working pressure will be at values of from 1 to 5 bar, frequently from 1.5 to 3.5 bar (in each case absolute). Normally, the working pressure in a heterogeneously catalyzed partial gas phase oxidation of an organic starting compound will not exceed 100 bar.
The reaction gas input mixture (or else reaction gas entry mixture) itself may, in the different procedures in the tube bundle reactor, be conducted either from the top downward or from the bottom upward in the reaction tubes (i.e. the at least one feed orifice may be disposed either in the upper reactor hood or in the lower reactor hood). The same applies to the conduction of the (preferably liquid) heat exchange medium.
The reaction gas input mixture may, on entry into the reaction tubes, in principle be preheated to the temperature of the heat exchange medium flowing on the corresponding tube plate underside.
The temperature of the reaction gas entry mixture, on entry into the reaction tubes, may, though, also be below this temperature of the heat exchange medium. This is advisable when the reaction tubes, in flow direction of the reaction gas mixture, are charged first with a longitudinal section of shaped bodies inert to the partial oxidation, before the catalytically active section of the fixed catalyst bed comprising shaped bodies having catalytically active composition begins. In the course of flow through this inert section, the reaction gas entry mixture may then be heated to the temperature of the heat exchange medium which flows around the corresponding catalytically active reaction tube section. In principle, the reaction gas entry mixture (the product gas mixture) can also be fed in (removed) via more than one feed orifice (removal orifice) present in the corresponding reactor hood. In general, though, both the feed of the reaction gas entry mixture and the removal of the product gas mixture are each effected via only one orifice in the corresponding reactor hood.
Frequently, a heterogeneously catalyzed partial gas phase oxidation of an organic compound can, in spatial terms, be connected immediately downstream of a heterogeneously catalyzed partial gas phase oxidation of another organic compound (in this case, the target product of the preceding partial oxidation is normally the organic compounds to be oxidized partially in the downstream partial oxidation) or connected upstream of it. In particular, in these cases, the feeding or removing reactor hood can be reduced to a cylindrical tube orifice (designed as a cylindrical tube opening), which may, for example, form a cylindrical transition to an aftercooler (cf., for example, DE-A 10 2004 018267 and DE 102007019597.6).
It will be appreciated that it is also possible to perform two heterogeneously catalyzed partial gas phase oxidations which are two successive gas phase partial oxidation steps in immediate succession in the reaction tubes of a multizone tube bundle reactor (for example in a two-zone tube bundle reactor), in which case the charge of the fixed catalyst bed in the reaction tubes of the multizone tube bundle reactor normally changes in a corresponding manner at the transition from one reaction step to the next reaction step (cf., for example, the performance of multistage heterogeneously catalyzed partial gas phase oxidations in the so-called “single reactor” according to EP-A 1388533, U.S. Pat. No. 6,069,271, EP-A 990636, US-A 2006/0161019 and EP-A 1106598). Examples of the performance of such multistage heterogeneously catalyzed partial gas phase oxidations in the multizone tube bundle reactor (for example two-zone tube bundle reactor) are the heterogeneously catalyzed partial gas phase oxidation of propylene to acrylic acid and of isobutene to methacrylic acid.
In addition to molecular oxygen and the organic starting compounds to be oxidized partially as reactants, the reaction gas input mixture of a heterogeneously catalyzed partial gas phase oxidation generally also comprises a diluent gas which behaves essentially inertly under the conditions of the heterogeneously catalyzed gas phase partial oxidation. In this document, this is understood to mean those diluent gases whose constituents, present in the reaction gas mixture, under the conditions of the heterogeneously catalyzed partial gas phase oxidation—each constituent taken alone—remain unchanged to an extent of more than 95 mol %, preferably to an extent of more than 99 mol %. They have the task firstly of absorbing some of the heat of reaction and conducting it out of the tube bundle reactor as a constituent of the product gas mixture, and secondly of ensuring that the reaction gas mixture is generally outside the explosion range. Inert diluent gases typically suitable for heterogeneously catalyzed partial gas phase oxidations of organic starting compounds are, for example, N2, CO2, steam, noble gases and in many cases also saturated hydrocarbons (for example in a partial oxidation of unsaturated organic compounds) or mixtures of all or of some of the aforementioned possible inert diluent gases.
The reactants present in the reaction gas mixture of a heterogeneously catalyzed partial gas phase oxidation (O2 and the organic starting compound) are converted as the reaction gas mixture passes through the fixed catalyst bed disposed in the reaction tubes during the residence time of the reactants over the catalyst surface.
The reaction tubes in the tube bundle reactor are, as already mentioned, generally manufactured from ferritic steel or from stainless steel and frequently have a wall thickness of a few mm, for example from 1 to 3 mm. Their internal diameter is usually a few cm, for example from 10 to 50 mm, frequently from 15 to 30 mm, or from 20 to 30 mm. The tube length extends normally to a few meters (a typical reaction tube length is in the range from 1 to 10 m, frequently from 2 to 8 m or from 2 to 6 m, in many cases from 2 to 4 m).
Appropriately from an application point of view, the number of reaction tubes accommodated in the tube bundle reactor is at least 1000, frequently at least 3000 or 5000 and in many cases at least 10 000. Frequently, the number of reaction tubes accommodated in the tube bundle reactor is from 15 000 to 30 000, or to 40 000, or to 50 000. Tube bundle reactors having a number of reaction tubes above 50 000 are usually the exception. Within the reaction tube surrounding space, the reaction tubes are normally arranged in essentially homogeneous distribution, the distribution appropriately being selected such that the distance of the central internal axes of mutually adjacent reaction tubes (the so-called reaction tube pitch) is from 25 to 55 mm, frequently from 35 to 55 mm.
Especially in the case of tube bundle reactors with a relatively large cross section of their tube plates, it is appropriate from an application point of view to leave a region without tubes in the center of the tube bundle reactor, and instead to support the upper tube plate within this region.
In principle, the total number of reaction tubes is distinguished into working tubes (the overwhelming majority of the reaction tubes) and into thermal tubes. While the working tubes are those reaction tubes in which the heterogeneously catalyzed partial gas phase oxidation in the actual sense is performed, thermal tubes primarily serve the purpose of monitoring and controlling the reaction temperature as a representative of the other reaction tubes (the working tubes). For this purpose, the thermal tubes, in addition to the fixed catalyst bed, normally comprise a thermowell which is conducted along the center of the thermal tube and is charged merely with a temperature sensor (for example a multithermoelement or an axially movable single thermoelement) (this is in many cases, but not necessarily, compensated for by an elevated internal diameter of the thermal tubes compared to the working tubes). In general, the number of thermal tubes in a tube bundle reactor is very much smaller than the number of working tubes. Normally, the number of thermal tubes is ≦20. In this context, it is of particular significance that the thermal tubes are charged with fixed catalyst bed such that the profile of the reaction temperature along the interior of a thermal tube corresponds very accurately to the profile of the reaction temperature along the interior of a working tube (cf. EP-A 873 783 and EP-A 1270 065).
The profile of the reaction temperature in the reaction tubes is determined firstly by the evolution of heat caused by the exothermicity of a heterogeneously catalyzed partial gas phase oxidation and secondly, inter alia, by the transfer of this heat of reaction to the at least one heat exchange medium conducted within the reaction tube surrounding space.
Since heterogeneously catalyzed partial gas phase oxidations are typically markedly exothermic reactions, and the heat of reaction is transferred to the at least one heat exchange medium at a finite rate, the temperature of the reaction gas mixture in the course of reactive passage thereof through the fixed catalyst bed is normally different from the temperature of the fluid heat exchange medium which flows around the fixed catalyst bed outside the reaction tubes. It is typically above the entrance temperature of the heat exchange medium THin into the corresponding reaction zone (temperature zone) and, along a reaction zone, generally passes through an absolute maximum (hotspot maximum) or falls proceeding from an absolute maximum value (if appropriate via further relative maxima). These maximum values of the reaction temperature (of the temperature of the reaction gas mixture) are typically referred to as so-called “hotspot temperatures”.
The hotspot temperature is therefore of particular significance because, where the reaction temperature in the reaction tube is elevated (the temperature of the fixed catalyst bed corresponds essentially to the temperature of the reaction gas mixture at the particular point), the irreversible aging processes in the fixed catalyst bed also proceed at an increased rate and cause accelerated deactivation of the fixed catalyst bed.
In this regard, it is known from the prior art that heterogeneously catalyzed partial gas phase oxidations in the reaction tubes of a tube bundle reactor which have been charged with a fixed catalyst bed can be performed over comparatively long periods (up to several years) in the case of careful operation without the fixed catalyst bed in the reaction tubes having to be renewed (freshly charged) (cf., for example, DE-A 10 350 822, DE-A 10 2004 025 445, EP-A 17 34 030 and the prior art acknowledged in these documents). The irreversible deactivation of the fixed catalyst bed is counteracted under otherwise essentially unchanged operating conditions typically by an increase in THin and/or an increase in the working pressure in the reaction tubes (cf., for example, EP-A 11 06 598, DE-A 10 351 269, EP-A 17 34 030, EP-A 990 636, DE-A 10 2004 025 445). These measures allow the target product space-time yield to be retained over prolonged operating times. However, they cause the aging process of the fixed catalyst bed to be accelerated further to an increasing extent (particular aging processes within the catalysts which contribute to aging proceed, for example, more rapidly). On attainment of a maximum value of THin, the fixed catalyst bed finally has to be exchanged completely (cf. also DE-A 10 232 748, EP-A 11 06 598 and DE-A 10 2007 010 422).
However, a disadvantage of such a complete exchange is that it is comparatively complicated. The process for target product preparation has to be interrupted for a prolonged period and the costs of catalyst preparation are likewise considerable.
What are likewise desired are therefore procedures which are helpful in as far as possible prolonging the lifetime of the fixed catalyst bed in the tube bundle reactor.
As already mentioned, the above is possible to a certain extent in the case of careful operation. Careful operation is understood in the prior art to mean operating the tube bundle reactor, within the context of what is possible, overall, such that, within the individual reaction tubes, as far as possible, a uniform reaction behavior and hence also a very uniform profile of the reaction temperature (of the temperature of the reaction mixture and of the temperature of the fixed catalyst bed) is present along the individual reaction tubes.
EP-A 14 71 046, DE-A 20 2006 014 116 U1 and WO 03/059857 recommend, in this regard, performing the heterogeneously catalyzed partial gas phase oxidation of an organic starting compound in tube bundle reactors whose reaction tubes are of very uniform construction.
According to the teaching of JP-A 2006-142288, the reaction tube inner surface should additionally have a very low surface roughness in order to ensure very uniform charging of the reaction tubes with fixed catalyst bed.
Such a very uniform charging of the reaction tubes with the same fixed catalyst bed is also recommended by the documents U.S. Pat. No. 4,701,101, EP-A 14 66 883, WO 03/057653, US-A 2006/245992, US-A 2002/136678, WO 2005/051532, WO 03/076373 and JP-A 2004/195279.
At the same time, it is quite generally attempted in heterogeneously catalyzed gas phase reactions to minimize the energy demand required for the conveying of the reaction gas. As a measure for achieving this objective, preference is given to using annular shaped catalyst bodies for the configuration of the fixed catalyst bed, since they cause a particularly low pressure drop in the course of passage of the reaction gas through the fixed catalyst bed (cf., for example, WO 2005/03039). A further advantage of annular shaped catalyst bodies normally consists in reduced diffusion pathways and, resulting from this in many cases, in an improved target product yield.
In the simplest case, such an annular shaped catalyst body consists only of catalytically active composition which may, if appropriate, be diluted with inert material (which is, for example, in many cases incorporated for reinforcement reasons) (if appropriate, shaping assistant is also present; for example graphite). Such annular geometric shaped catalyst bodies are typically referred to as annular unsupported catalysts.
However, a disadvantage of annular unsupported catalysts is their generally not fully satisfactory mechanical stability in the course of filling into the reaction tubes. Although this can be improved by an increase in their wall thickness, a disadvantage of relatively large wall thicknesses is that they are accompanied by a lengthening of the diffusion pathway out of the reaction zone, which promotes undesired subsequent reactions and hence reduces the target product selectivity.
A resolution of the contradiction which exists in the case of unsupported catalyst rings between required mechanical stability (increasing wall strength) on the one hand and limiting of the diffusion pathway out of the reaction zone (decreasing wall strength) on the other hand, while maintaining the otherwise particularly advantageous ring geometry, is opened up by annular coated catalysts. These are annular shaped catalyst bodies which consist of an annular (mechanically particularly stable) (catalytically inactive) shaped support body which is generally inert with regard to the gas phase partial oxidation and a catalytically active composition (active composition) applied to its surface.
It can be prepared, for example, by coating the annular shaped support bodies (generally consisting of catalytically inactive (frequently oxidic (e.g. hard-fired)) material; consisting of inert material) with finely divided active composition using a generally liquid binder. Alternatively (or in a mixture with finely divided precursor composition), the shaped support bodies may also be coated with a finely divided precursor composition of the active composition using a generally liquid binder, and the conversion to the active annular shaped catalyst bodies can be effected by subsequent (for example oxidative and/or reductive) thermal treatment (if appropriate in an atmosphere comprising molecular oxygen). The coating can be effected in the simplest manner, for example, by moistening the surface of an inert annular shaped support body (or else simply just “support body”) by means of a liquid binder and then adhering finely divided (pulverulent) active composition or finely divided (pulverulent) precursor composition on the moistened surface. Subsequently, normally at least a portion of the liquid binder (generally under the action of heat) is volatilized before the annular coated catalysts are ready to charge a reaction tube (a further thermal treatment can be effected, for example, within the reaction tubes (for example for the purpose of removing residual binder [cf., for example, DE-A 102005010645])). Alternatively, the annular shaped support bodies can also be sprayed with a suspension of finely divided active composition and/or finely divided precursor composition.
Instead of coating the generally inert annular shaped support body with finely divided active composition or with finely divided precursor composition, the annular shaped support body can in many cases also be impregnated with a solution (a molecular and/or colloidal solution) of the catalytically active substance or with a solution of a precursor substance and then the solvent can be volatilized and, if appropriate, a chemical reduction and/or thermal treatment (if appropriate in a molecular oxygen-comprising atmosphere) can follow. The annular shaped catalyst bodies which result in this way are frequently also referred to as supported or impregnated catalysts in the literature. However, they will likewise be encompassed in this document under the generic term “coated catalysts”.
Descriptions of processes for preparing annular coated catalysts which are suitable as catalysts for heterogeneously catalyzed partial gas phase oxidations can be found, for example, in the documents DE-A 290 9671, EP-A 714 700, German application 102007017080.9, WO 2004/108267, DE 10 2005 010 645 A1, DE-A 103 13 209, DE-A 103 25 488, DE-A 103 60 058, DE-A 103 51 269, DE-A 103 50 822, WO 2007/009922, DE-A 100 49 873, German application 102007010422.9, DE-A 40 06 935, DE-A 198 23 275, DE-A 198 39 001, DE-A 198 23 262, DE-A 103 44 844, US 2006/0205978 and EP-A 758 562, and the prior art acknowledged in these documents.
An unwanted by-product, which, though, generally cannot be avoided completely, in the preparation of annular coated catalysts is the formation of adhering pairs of annular coated catalysts. These are two coated catalyst rings which adhere firmly to one another. Their formation is attributable ultimately to the fact that the normally liquid binder typically used to apply the active composition coating to the annular support body in the preparation of coated catalysts is capable of bringing about not only the bonding of active composition and shaped support body but, to a limited degree, also the unwanted bonding of two annular coated catalysts. Essentially, the formation of such adhering pairs is restricted to two types: a) fused adhering pairs and b) tandem adhering pairs.
In the fused adhering pairs, two annular coated catalysts (an annular coated catalyst has the geometry E×I×H (external diameter×internal diameter×height)) adhere to one another by their cylindrical shells (outer walls) essentially over the entire height H. They essentially adhere to one another resting alongside one another at the same height (their outer surfaces adhere to one another).
In the tandem adhering pairs, two annular coated catalysts adhere to one another by their annular cross-sectional areas which delimit the particular coated catalyst ring at the top and at the bottom. The upper ring surface of one coated catalyst ring adheres on the lower ring surface of one coated catalyst ring adheres (sticks) on the lower ring surface of the other coated catalyst ring. In this way, what is effectively formed is a coated catalyst super-ring which has the same external diameter E and the same internal diameter I as the two coated catalyst rings which constitute it, but whose height is 2 H.
While the formation of tandem adhering pairs in the preparation of annular coated catalysts is essentially unavoidable, fused adhering pairs form in the preparation of annular coated catalysts essentially (in particular) when H is at least >0.5 E.
Overall, the total amount M of adhering pairs of coated catalyst rings formed in the preparation of one production charge of annular coated catalysts, based on the total weight of the production charge, is ≦5% by weight. Usually, M, on the same basis, is even ≦4, or ≦3, or ≦2, or ≦1% by weight. In the case of careful preparation of annular coated catalysts, M, on the same basis, may even be ≦0.8% by weight, or ≦0.5% by weight, or ≦0.3% by weight, ≦0.2% by weight, or ≦0.1% by weight. In general, M is, however, on the same basis, >0, usually ≧0.005 and frequently even ≧0.01% by weight.
Owing to the aforementioned comparatively low amounts of adhering pairs of coated catalyst rings formed, no increased significance was attributed to their presence in the use of production charges of annular coated catalysts for the configuration of the fixed catalyst bed in the reaction tubes of tube bundle reactors.
However, extremely careful investigations by the applicant with regard to the configuration of fixed catalyst beds in reaction tubes of tube bundle reactors using annular coated catalysts have led to the result that, in the case of an unfavorable position of an adhering pair of coated catalyst rings in the fixed catalyst bed disposed in a reaction tube of a tube bundle reactor, the hotspot temperature in this reaction tube can be increased perceptibly merely by the presence of a single adhering pair of coated catalyst rings in the fixed catalyst bed.
However, an elevated hotspot temperature means accelerated aging of the corresponding fixed catalyst bed charge of a reaction tube. In order to compensate at least temporarily for such an accelerated aging process with regard to the desired space-time yield of target product in a heterogeneously catalyzed partial gas phase oxidation performed in a tube bundle reactor, an accelerated increase of THin of the at least one heat exchange medium is required, which in turn causes an additional acceleration of the aforementioned aging process. The ultimate overall effect which results is a reduced lifetime of the fixed catalyst bed charge of the tube bundle reactor, which is undesired for the reasons already described.
The presence of one (or else more) adhering pair(s) of coated catalyst rings is particularly disadvantageous in the fixed catalyst bed of a thermal tube. The profile of the reaction temperature within the thermal tubes arranged in the tube bundle reactor to be representative of all working tubes forms, as already stated, the basis for the control of the overall operation of a tube bundle reactor (for example the control of the loading of the fixed catalyst bed with reaction gas, the control of the composition of the reaction gas mixture, the setting of the particular THin etc).
In general, the control of the overall operation, for safety reasons, is directed to those thermal tubes whose operating data are the most marginal. When these operating data are representative of the operating data of the corresponding working tubes only to a limited degree owing to the presence of adhering pairs of coated catalyst rings in the fixed catalyst bed of the corresponding thermal tubes, this normally leads to the effect that the overall tube bundle reactor is not operated in its optimal operating state.